The invention is in the general field of electrodes for amperometric biosensors. More specifically, the invention is in the field of compounds for use as mediators for the recycling of cofactors used in these electrodes.
NAD- and NADP-dependent enzymes are of great interest insofar as many have substrates of clinical value, such as glucose, D-3-hydroxybutyrate, lactate, ethanol, and cholesterol. Amperometric electrodes for detection of these substrates and other analytes can be designed by incorporating this class of enzymes and establishing electrical communication with the electrode via the mediated oxidation of the reduced cofactors NADH and NADPH.
NAD- and NADP-dependent enzymes are generally intracellular oxidoreductases (EC 1.x.x.x). The oxidoreductases are further classified according to the identity of the donor group of a substrate upon which they act. For example, oxidoreductases acting on a CH—OH group within a substrate are classified as EC 1.1 .x.x whereas those acting on an aldehyde or keto-group of a substrate are classified as EC 1.2.x.x. Some important analytes (e.g., glucose, D-3-hydroxybutyrate, lactate, ethanol, and cholesterol) are substrates of the EC 1.1.x.x enzymes.
The category of oxidoreductases is also broken down according to the type of acceptor utilized by the enzyme. The enzymes of relevance to the present invention have NAD+ or NADP+ as acceptors, and are classified as EC 1.x. 1.x. These enzymes generally possess sulphydryl groups within their active sites and hence can be irreversibly inhibited by thiol-reactive reagents such as iodoacetate. An irreversible inhibitor forms a stable compound, often through the formation of a covalent bond with a particular amino acid residue (e.g., cysteine, or Cys) that is essential for enzymatic activity. For example, glyceraldehyde-3-P dehydrogenase (EC 1.2.1.9) is stoichiometrically alkylated by iodoacetate at Cys149 with concomitant loss of catalytic activity. In addition, the enzymes glucose dehydrogenase, D-3-hydroxybutyrate dehydrogenase (HBDH), and lactate dehydrogenase are known to be irreversibly inhibited by thiol reagents. Thus, in seeking to develop stable biosensors containing NAD- or NADP-dependent dehydrogenases, avoidance of compounds that are reactive toward thiols is imperative, as they can act as enzyme inhibitors.